1. Field of the Invention
This invention relates generally to universal serial bus utilization, and more particularly to low power universal serial bus utilization during the unconfigured state of a USB-to-USB device.
2. Description of the Related Art
In recent years, the universal serial bus (USB) has come into being as an interface for easily connecting a plurality of peripheral devices to a personal computer. Consequently, peripheral devices having a port for USB (hereinafter referred to as a USB port) have become increasingly available.
Prior to USB, in order to connect a keyboard, printer, monitor, modem or mouse to the personal computer, it was often necessary to use several different types of local electrical buses and associated ports, such as a serial port for a modem, a parallel port for the printer, a keyboard port and a mouse port. This leads to unnecessary complexity since each peripheral device may use a different bus. Therefore, USB was created to provide a standard interconnect for peripherals, and to facilitate connecting peripheral devices to the computer. USB not only replaces the multiple cables and physical connectors typically needed with a single standardized connection system, it provides a standard electrical specification.
USB also permits peripheral devices to be connected and/or disconnected from the bus while the computer system is powered up which eliminates the need, with conventional buses, to power down and “re-boot” every time that a peripheral device is connected or disconnected. In addition, USB permits a peripheral connected to the USB to be detected and a configuration process for the device, known as enumeration, may be commenced.
The employment of USB enables the use of common interfaces for mice, keyboards, printers, modems, speakers, joy sticks, etc. Further, equipping a personal computer with a single USB port enables up to 127 peripheral devices to be connected to each other using a USB hub apparatus.
USB supports two types of power sourcing capabilities: low-power ports and high-power ports. Low-power ports are capable of delivering a maximum current of 100 mA to a USB device, while high-power ports are capable of delivering a maximum current of 500 mA to a USB device. USB also supports two types of bus-powered devices: low-power bus-powered devices and high-power bus-powered devices. Low-power bus-powered devices are capable of drawing a maximum current of 100 mA from a USB bus, while high-power bus-powered devices are capable of drawing a maximum current of 500 mA from a USB bus.
Also supported in the USB specification are self-powered devices. Self-powered devices are capable of drawing a maximum current of 100 mA from a USB bus to allow the USB to function when the rest of the device is powered down. All other power requirements for the device are provided from external power sources.
As mentioned above, USB is designed to support “Plug-and-play,” which provides ability to add new USB components and have them work without needing to perform any technical analysis or procedure on the USB component. As a result, it is common for USB devices to be interchanged regularly. As will be appreciated by those skilled in the art, connecting a high-power bus-powered device to a low-power port can cause compatibility problems since the device will require 500 mA from the low-power port, which can only supply 100 mA.
To address this issue, the USB specification requires USB devices to draw a current of less than 100 mA per connecting USB port during an unconfigured state. If the device requires a higher current for full functionality, such as a high-power bus-powered device, the device negotiates for higher current by reporting its current requirements through device descriptors.
USB-to-USB devices have two USB upstream ports, each of which is connected to a USB downstream port of a host computer. In this manner, two host computers can communicate with each other using the USB-to-USB device. This USB-to-USB connection allows the host computers to share resources such as hard drives, CD-ROMs, printers, and network access.
In a USB-to-USB device, since two USB ports are connected to the same device, the current drawing from both USB ports during the unconfigured state can total up to 200 mA. Unfortunately, prior art USB devices often are not designed to limit power from multiple USB ports during the unconfigured state of a device.
In view of the foregoing, there is a need for systems and methods that limit current drawn from each port of a multi-port USB device. The systems and methods should limit current drawn from each port to a maximum of 100 MA per port.